1. Field of the Invention
The present invention relates to rotary shear cutting of corrugated paperboard. More specifically, the invention relates to the fabrication of rotary shear knives used to slit and slot corrugated paperboard box blanks.
2. Description of the Prior Art
In the process of converting corrugated paperboard to containers of various sizes and shapes, blank sheets of the material are conveyed serially through the nips of one or more rotary cutting dies which shear strips of corrugated material to leave slot shaped voids in the sheet blanks. These slots, cut by the die engagement, define flaps which, when folded 90.degree., form the top and bottom container surfaces. A mere slit cut between adjacent flaps is insufficient due to physical interference between adjacent flap edges. As a general rule, such flap defining slots cut into a blank sheet are about 1/16" wide.
Due to the construction of corrugated paperboard as having parallel facing or liner sheets held together in spacial separation by an undulated medium sheet, clean and precise cuts are difficult to achieve. In those cases where crushing of the corrugated medium flutes along the cut edge is intolerable, the board may be cut by sawing. A saw-cut generated kerf, however, is wasteful of board fiber and when applied to large production rates, generates intolerable quantities of fiber dust.
More frequently, localized medium crushing along a cut perimeter may be tolerated to permit other cutting techniques. Among the other techniques is the shear die cut whereby two knife edges aligned in the same plane shear together upon the corrugated board positioned across the common plane between the knife edges. If applied to a continuous line of severance, the two knife edges may take the form of two sharp-edged discs having a slight perimeter overlap. For cutting slots and kerfs, two parallel shear planes are severed by a double edged male knife which meshes with a pair of corresponding knife edges in the form of a unitized female knife. Male and female knives of a given set are mounted on respective carrier rolls which are synchronously driven, rotatively.
Although papermakers expend considerable capital and process effort to remove foreign abrasive substances from paper pulp prior to web forming, success is achieved only in degrees less than total. Such abrasives remaining in dry paper webs are largely responsible for knife edge frictional heating and dulling which necessitates removal of the knife dies from respective carrier rolls for edge grinding. However, because of small initial meshing lap dimensions, such edge regrinding is permissible only two or three times at most before a sufficient amount of knife material is removed to prevent edge meshing.
Another cause of knife edge dulling is the customary close dimensional tolerance in the mesh fit between male and female knife edges. Zero tolerance is the desired objective. When the knives are adjusted to machine position, however, the machine is static and inoperative. Subsequent operational forces and vibrations create dynamic mismatches between respective knife edges when they are driven together. Consequent edge clashing and highly loaded, lapped sliding also generates considerable heat within the knife bodies which expand in further aggravation of the dulling result.
It is, therefore, an objective of the present invention to provide a rotary die shear design by which knife edges may be convectively air cooled.
Another object of the invention is to provide air cooled knife edges for rotary die shears that are conveniently removable and replaceable without disturbing relative machine alignments.
Another object of the invention is to provide conveniently removable knife edges that, in assembly, are intimately associated with highly convective heat transfer surfaces.